This invention relates to thin-film-like particles of a very small thickness having a skeleton constructed by carbons, and monolayer and stacked-layer very thin isolated films composed of the particles.
In recent years, search for a substance highly anisotropic in shape and its applications have progressed rapidly. A composite of such a substance, gathered in a large number, with other substance can be expected to exhibit various properties, such as high strength, when added at a low fraction. If the substance has a very fine linear (one-dimensional) or very thin planar (two-dimensional) shape and is electrically a semiconductor or a conductor, the substance used alone, or a collective matter of a small number of the substances, can be expected to show a quantum effect in electronic properties.
Among substances having carbon atoms as a skeleton and having an anisotropic shape, a graphite fiber and a carbon nanotube, which is a particularly fine form of the graphite fiber, are known as one-dimensional substances, while graphite, graphite fluoride and graphite oxide are known as two-dimensional substances. Of these substances, graphite, graphite fluoride and graphite oxide are all multi-layer structure materials comprising two-dimensional fundamental single layers stacked, and the number of the layers is generally very large. The fundamental layer of graphite (the fundamental layer is called graphene, and composed of carbons alone) comprises carbons of sp2 bonds, and has a structure of one carbon atom thick (0.34 nm). The fundamental layer of graphite fluoride has a structure comprising a sp3-bonded carbon skeleton of one or two carbon atoms thick counted as rows of diamond-like zigzag carbons, and fluorine bonded to both surfaces of the skeleton. The fundamental layer of graphite oxide is assumed to have a structure comprising a carbon skeleton composed mainly of sp3 bonds with a slight tendency toward sp2 bonds and similarly one or two carbon atoms thick counted as rows of zigzag carbons, and acidic hydroxyl groups or the like bonded to both sides of the skeleton (the structure is 0.61 nm thick, if the thickness of the carbon skeleton is equal to the dimension of one carbon atom, hydroxyl groups, etc. are present on both sides of the skeleton, and intercalated water is very little) (for example, xe2x80x9cGraphite Intercalation Compoundsxe2x80x9d, Chapter 5, edited by Carbon Materials Association, Realize Co. (1990); T. Nakajima et al., Carbon, 26, 357 (1988); M. Mermoux et al., Carbon, 29, 469(1991)). When the graphite oxide is highly oxidized and thoroughly dried, its oxygen content is about 40 wt. %.
The electronic structure of graphite and that of graphene are theoretically known to differ slightly. Graphene does not occur naturally, and there is an example of its synthesis as a film deposited on the surface of a nickel crystal by CVD (E. Rokuta et al., Surf. Sci., 427-428, 97(1999)). However, there has been no example of graphene actually prepared as an isolated thin film.
Examples in which such multi-layer structure materials having a carbon skeleton are separated into many fundamental layers include a material having isoprene or the like polymerized in interlayer spaces of graphite (H. Shioyama, Carbon, 35, 1664(1997)), a material having polyethylene oxide penetrating interlayer spaces of graphite oxide (Y. Matsuo et al., Carbon, 34, 672(1996)), and a material having aniline or the like polymerized in interlayer spaces (Japanese Unexamined Patent Publication No. 1999-263613).
In these examples involving separation of the multi-layer structure, the fundamental layers or very thin layers close to them are only existent as constituent components inside the composite, and have not been withdrawn separately and stably. That is, very thin-film-like particles, which have a carbon skeleton with high crystallinity and which can exist as independent particles, have not been discovered. An isolated film that is formed by the linkage of the separated thin layers has not been formed.
The object of the present invention is to provide such thin-film-like particles and an isolated films relatively similar to graphene, but different in structure from graphene.
To attain the above object, the inventors of the present invention selected the graphite oxide, in which separation of the layers can be expected to occur relatively easily, from the aforementioned three multi-layer structure materials, and further performed its synthesis (oxidation and purification) for promoted separation of the layers, thereby obtaining the desired thin-film-like particles. The structure of the thin-film-like particle is practically equal to the structure of graphite oxide so far known, unless it is very thin. However, the thin-film-like particle has a hitherto unknown, very thin shape, namely, a shape with a very small thickness relative to a breadth in its planar direction. When expressed as the number of layers within the particle, the thickness is less than 20 times the thickness of the fundamental layer. Consequently, the thin-film-like particle can even deform flexibly, although it has a dense carbon skeleton.
The thin-film-like particles are desirably handled as a dispersion in a liquid. However, studies were conducted not only of water, a dispersion medium immediately after synthesis, but also of replacement by other dispersion media. Through these studies, the inventors facilitated applications to composing of the thin-film-like particles with other substances. Furthermore, they made it possible to reduce the thin-film-like particles into thin-film-like graphite particles with a very small thickness and having a nearly graphite-like structure, or a collective matter of the thin-film-like graphite particles, as is known with ordinary graphite oxide.
Besides, the particles in the dispersion can be placed, along with the dispersion, on a mesh, and dried, whereby isolated thin films out of contact with the substrate can be obtained by the linkage of thin-film-like particles.
Graphite with a well-developed layer structure and high crystallinity is desirable as a raw material for the thin-film-like particles of the present invention. In such graphite, the respective fundamental layers are large, and the frequency of existence of a bonds tying the adjacent two fundamental layers together is extremely low. Thus, the graphite is liable to separate into thin-film-like particles after an oxidation reaction. With graphite having an undeveloped layer structure and low crystallinity, by contrast, oxidation occurs, but separation of the layers is extremely difficult. More concretely, desirable graphite is one in which the diameter of the widest fundamental layer within the particle is nearly equal to the diameter of the particle, and the entire particle has a single multi-layer structure. Known examples of such graphite are natural graphite (especially, of a high quality), kish graphite (especially, one produced at high temperatures), and highly oriented pyrolytic graphite. The respective fundamental layers of natural graphite and kish graphite are each an single crystal having a nearly single orientation, while the respective fundamental layers of highly oriented pyrolytic graphite are each a collective matter of many small crystals having different orientations. In the present invention, any of these graphites, or exfoliated graphite having the interlayer spaces of these graphites broadened beforehand is used as the starting material.
The size of the fundamental layer of graphite, and the size of a minute part within the fundamental layer can be estimated from the shapes of peaks in X-ray diffraction, by observation of an electron channeling contrast image under a scanning electron microscope, or by observation under a polarization microscope. Other indicators include, for example, an electric resistance of about 10xe2x88x926 xcexa9m or less. However, such indicators only show the possibility for separation of the layers. Actually, therefore, it is desirable to perform oxidation and purification using the intended graphite material and confirm the separation of a multi-layer structure into individual layers.
Impurities in the graphite, such as metal elements, are desirably decreased to a content of about 0.5% or less beforehand. If the content of the impurities is high, separation of the multi-layer structure into the layers may be impaired.
The particle diameter of graphite is reflected in the size in the planar direction of the resulting thin-film-like particle, and thus may be selected according to the size of the thin-film-like particle to be synthesized. Thin-film-like particles having a breadth of several millimeters or more can be essentially synthesized. However, as the diameter of the particle increases, the time required for oxidation lengthens. If it is desired to define the shape in the planar direction of the resulting thin-film-like particle, for example, as a square, the starting graphite may be cut beforehand to a square shape. However, the orientation of the crystal needs to be recognized at the time of cutting.
For oxidation of graphite in the present invention, there can be employed the publicly known methods, such as the Brodie method (using nitric acid and potassium chlorate), the Staudenmaier method (using nitric acid, sulfuric acid, and potassium chlorate), and the Hummers-Offeman method (using sulfuric acid, sodium nitrate, and potassium permanganate). Of these methods, the Hummers-Offeman method (W. S. Hummers et al., J. Am. Chem. Soc., 80, 1339(1958); U.S. Pat. No. 2,798,878 (1957)) particularly results in the higher degree of oxidation. This method of oxidation is particularly recommended in the present invention.
With any-of these methods for oxidation of graphite, ions of an oxidizing agent first penetrate into interlayer spaces of the graphite to form an intercalation compound. Then, water is added to hydrolyze the intercalation compound, forming graphite oxide. Of these reactions, the formation of the intercalation compound particularly takes time, and depends on the particle diameter of graphite. Hence, it is desired to vary the time for coexistence with the oxidizing agent according to the particle diameter of graphite, and allow as many ions of the oxidizing agent as possible to penetrate the interior of the graphite particles. Investigation by the inventors showed the penetration of about 10 xcexcm or more of ions per hour at a temperature of about 20xc2x0 C. according to the Hummers-Offeman method. In this view, it is desirable to oxidize graphite for an oxidizing time of at least 30 minutes, if possible, 3 hours or more, for a particle diameter of 10 xcexcm of graphite.
According to the foregoing methods of oxidizing graphite, it is necessary to purify the product by removing the oxidizing agent remaining in the reaction mixture, or ions resulting from decomposition of the oxidizing agent, or components derived from the ions. With the publicly known method for oxidation, this purification is performed by washing with water or alcohol. In the present invention, components, which may remain in the reaction mixture or in the interlayer spaces to hinder separation of the layers, are removed more aggressively at this purification stage to promote separation into thin-film-like particles. That is, low molecules and small ions coexisting in the liquid, other than the dispersion medium, are maximally removed, thereby increasing the degree of ion dissociation of the acidic hydroxyl groups present in the respective layers of graphite oxide, and enhancing electrostatic repulsion among the respective layers deemed to be ionic large particles, to promote the separation of the multi-layer structure into the layers.
Investigation by the inventors showed that when the concentration of sulfuric acid was about 0.05 wt. % or less, for example, at a graphite oxide concentration of about 1 wt. % or less, separation of the multi-layer structure into the layers proceeded promptly. Assuming that the ion dissociation of sulfuric acid occurs up to one stage, the concentration of small ions other than those derived from graphite oxide in the reaction mixture (including hydrogen ions formed by ion dissociation of graphite oxide) is calculated at about 10 mol/m3 or less. Thus, the product is desirably purified until this concentration or lower is reached. Generally, as this purification is progressed, separation of the layers proceeds. Concretely, after water is added, water is removed together with the small ions. The water used is desirably of a high purity.
To proceed with the separation of the respective layers which are ionic large particles, on the other hand, it is important to lower the concentration of graphite oxide particles in the liquid during purification and raise the degree of ion dissociation of the respective layers. Thus, the concentration of graphite oxide at a stage, where the particles have been dispersed uniformly upon addition of water, is set at about 5 wt. % or less, more desirably 1 wt. % or less.
With the Hummers-Offeman method, usually, hydrogen peroxide is added after hydrolysis to decompose permanganate ions into manganese (IV) ions, whereafter the manganese ions are removed by washing with water along with other sulfate ions and potassium ions (W. S. Hummers et al., J. Am. Chem. Soc., 80, 1339(1958)). However, if the system becomes neutral, the solubility of manganese ions may lower and turn into manganese hydroxides, remaining between the layers. Hence, before washing with water, it is desirable to do thorough washing with an aqueous solution of sulfuric acid or a mixed aqueous solution of sulfuric acid and hydrogen peroxide.
A concrete purification operation by washing may employ publicly known means such as decantation, filtration, centrifugation, dialysis or ion exchange. As the particle diameter of the starting graphite decreases, or as the separation of the layers proceeds to increase thin-film-like particles, or as the removal of small ions progresses, electric charges per unit volume of the thin-film-like particles increase. As a result, repulsion between the particles strengthens, and the degree to which the dispersion medium is retained (for water, the degree of hydration) also heightens, thus making any purification operation difficult. In this case, the operation with a relatively high purification efficiency is centrifugation, dialysis, or ion exchange. In particular, the operation capable of purification in a relatively short time is centrifugation. On the other hand, decantation or filtration is slow in sedimentation or undergoes blockade due to thin-film-like particles, thus posing more difficulty as the diameter of the thin-film-like particles decreases. To lower the repulsion between the particles temporarily, the use of other solvent with a low dielectric constant, or salting-out may be suitably combined with the purification operation.
During purification, the separation of the multi-layer structure into the layers occurs spontaneously. Besides, a stirring operation, such as shaking, is added during formation of a uniform dispersion upon re-adding of water after removal of water together with small ions, so that the separation is further promoted. Ultrasonication is also usable, but tends to break the fundamental structure of the respective layers into small structures according to the separation of the layers, and so is desirably used when it is desired to form thin-film-like particles with a particularly small diameter.
Purification in the above-described manner promotes the separation of the layers inside many particles. However, particles, which do not take a thin-film-like shape and in which the separation of the multi-layer structure into the layers is insufficient, also remain in small amounts. These particles are impurities in the starting material (i.e., graphite difficult to separate, and other inorganic substances), and foreign bodies incorporated during oxidation or purification. These particles generally are apt to sediment, and thus can be removed by decantation or very slow centrifugation during purification.
The above procedure promotes the separation of the layers within many particles. The possibility for separation rises even in the layers, which have not been separated. However, since the particles are big, many hydrogen bonds, etc. may be present in the interlayer spaces within the particles, thus making separation in a short time substantially difficult. A method for further promoting separation of the layers would be to dilute the dispersion after completion of purification, and then strengthen the molecular movements of the dispersion medium or the movements of thin-film-like particles. Concretely, the method includes, for example, ultrasonication or heating of the dispersion. However, ultrasonication, as stated earlier, tends to destroy and divide the fundamental structures of the respective layers in accordance with the separation of the layers. With heating, the degree of ion dissociation can be expected to become high. However, at particularly high temperatures, the particles are likely to be partially reduced, so that heating is desirably performed at not so high temperatures. Concretely, the temperature is 50 to 150xc2x0 C.
To selectively obtain particles, in which the separation of the layers proceeds further, fractionation according to differences in dispersibility is recommendable. For example, it is advisable to perform decantation or relatively slow centrifugation and use non-sedimentation parts.
The above-described respective operations accomplish a dispersion of thin-film-like particles with a very small thickness, which can be called nanofilms, dispersed in water.
When this dispersion of the thin-film-like particles, like common graphite oxide, is dried at a high concentration, many particles aggregate and become difficult to disperse again. (Many studies hitherto done on the structures of graphite oxides have been on solids in the aggregated state, and such thin-film-like particles as in the present invention have not been known.) In using the thin-film-like particles for concrete purposes, therefore, it is desirable to store and handle them, if possible, in the as-dispersed state, to obtain minimally aggregated thin-film-like particles from a very low concentration dispersion by drying, spray drying, or freeze drying, or to use them in the dispersed state and mix them with other substance.
When the thin-film-like particles are used in the as-dispersed state, a dispersion medium other than water may be desirable depending on uses. In this case, it is recommendable to replace the present dispersion medium with the other dispersion medium by using the other dispersion medium during the above purification, or by repeating the step of concentrating the dispersion by centrifugation or the like after purification to decrease the water, then adding the other dispersion medium, mixing the system, and then concentrating the mixture by centrifugation or the like. The thin-film-like particles have high polarity, and thus show high affinity for a polar dispersion medium with a high dielectric constant. Thus use of such a dispersion medium results in minimal aggregation of the thin-film-like particles. Concretely, a dispersion medium having a relative dielectric constant of about 15 or more is desirable. If, at the time of replacement of the dispersion medium, the two dispersion media are poorly miscible, replacement may be performed via a third dispersion medium with satisfactory miscibility with both of the two dispersion media.
The thin-film-like particles obtained in the present invention have functional groups such as hydroxyl groups. Thus, their reaction with, for example, formaldehyde, carboxylic acids, isocyanates, and epoxides can be expected. In this case, if other molecules to be reacted with the thin-film-like particles have a plurality of functional groups or a functional group forming a plurality of bonds, these molecules crosslink a plurality of thin-film-like particles.
When the thin-film-like particles obtained in the present invention are mixed with other organic or inorganic polymerizable material and the polymerizable material is polymerized, a composite containing the thin-film-like particles can be formed. In this case, a dispersion of the thin-film-like particles is mixed with the other polymerizable material, and the polymerizable material is polymerized after removal of the dispersion medium, aggregation of the thin-film-like particles in the composite can be minimized.
When a methanol dilution of the thin-film-like particles obtained in the present invention is dropped onto a metal mesh, and the liquid is evaporated, a dry thin film forms. The particles in the liquid are at a low concentration and minute, but they are tied to each other during drying to form thin films integrated so as to cover not only surface of the mesh but also its opening portions. The thin films formed at the opening portions of the mesh are in an isolated state. These isolated films can also be called nanofilms.
For observation under a transmission electron microscope (TEM), a sample must be maintained stably. Thus, a carbon microgrid (here, a microgrid corresponds to a microporous mesh) having many small openings is laminated to the above-mentioned metal mesh, and a thin film is prepared on the laminate by the same manner as described above. The resulting product is inserted into the TEM, and very thin regions where electron beams are clearly transmitted are observed. The morphology, thickness, micrographic structure, crystallographic structure, composition, and electronic state are analyzed through image observation, observation of the diffraction pattern, and observation of the electron energy loss spectroscopy spectrum.
If an electronic nature is required to the thin-film-like particles obtained in the present invention, it is desirable to reduce the thin-film-like particles into an electronic state composed mainly of sp2 bonds similar to graphite, thereby enhancing electric conductivity. For reduction, various known reduction reactions or electrode reactions using reducing agents (i.e., electrolytic reduction) can be used. When a reducing agent is used, in particular, complete reduction up to the interior of multi-layer particles may be difficult, unless decomposition can be extended up to the fundamental layers. As a general behavior of graphite oxide, it is known that a structure resembling graphite can be formed, by heating, as far as the interior of the multiple layers; and that upon heating with a plurality of particles being in an aggregated state, intermolecular forces occur in interlayer spaces inside the multi-layer particles or between the plural particles, whereby a macroscopic shape, such as an ordinary graphite film, can be imparted (J. Maire et al., Carbon, 6, 555(1968)). The thin-film-like particles of the present invention have a particularly thin shape. Thus, when given a graphite-resembling structure by similar heating, the thin-film-like particles turn into thin-film-like graphite particles, which can be called carbon nanofilms or graphite nanofilms. Such thin-film-like graphite particles (singular form), or larger film-like structure materials (collective matter) comprising a plurality of the thin-film-like graphite particles aggregated in a planar form, are expected to show a two-dimensional quantum effect in the electronic nature or the like. For concrete uses, for example, the thin-film-like particles may be placed on a suitable substrate with high resistance to heat, and reduced by heating, whereafter the resulting thin-film-like graphite particles may be worked into a predetermined shape by various etching methods, etc.
The thin-film-like graphite particles can be mixed with other polymerizable material, and the polymerizable material can be polymerized to form a composite containing the thin-film-like graphite particles. Electric conductivity, for example, can be imparted to the composite.
The thin-film-like graphite particles can also become precursors for novel carbon structure materials, such as thin-film-like diamond and thin-film-like large hydrocarbon.
The thin-film-like particles obtained in the present invention are thin structure materials having a dense carbon skeleton. Thus, the particles in an singular form, or larger film-like structure materials comprising a plurality of the particles aggregated in a planar form, including the particles in reduced form, can become film materials having selective permeability or shielding properties to elementary particles, such as muons or protons, small ions, and low molecules.